Druid at level 6 will make the water less of an obstacle, so I would recommend throwing some beasties in there to take on a wildshaped druid at their level, especially if said druid has "natural spell." .

Wait, I'm confused. There are druids who DON'T take the Natural Spell feat?

EDIT: In all seriousness though, throw them a light curveball. Giant leeches for example, or an incorporeal enemy.

Warmage is fairly easy to counter, just take a look at what spells the Warmage has available. One thing to consider is if that Warmage didn't bother with any acid spells, to keep that scrag under water as long as possible, since above the water the scrag will be incredibly vulnerable to a warmage's fire attacks.

Warmages don't have to "bother" with spells, in the same way that Sorcerors don't; they're spontaneous casters. With

They have the Advanced Learning class feature; at certain levels, it lets them add to their list of spells known.

very minor deviations, all warmages have pretty much the same narrow spell selection (primarly focused on blasting). When they gain access to a new level of spellcasting, they automatically know all warmage spells of that level, in the same way that a cleric or druid would know their respective spells.

The warmage has fireball, scorching ray, and acid arrow. He also has lesser orb of acid, which probably presents the greatest threat to Wetsig.

As mentioned, the druid can wild shape. Even if he's not optimized, he still has spontaneous Summon Nature's Ally spells, which can include aquatic creatures. He also has access to Dominate Animal, so any animals the scrag has trained might be forced to fight for the party instead.

The barbarian is big and strong, but unless he has a piercing weapon, he'll be taking penalties for fighting underwater. This sort of fight should be entertaining for him.

The monk is... a monk. Unless he's crazy optimized, this should be challenging for him unless he gets lucky with a Stunning Fist.